This invention relates to a surface light source device of side-light type adopting a light guide plate with emitting directivity and a propagation direction characteristics modifier in combination. The surface light source device of the present invention is effectively applicable to, for instance, a back lighting arrangement in a liquid crystal display requiring an illumination flux concentratively propagating to a frontal direction.
A well-known surface light source device of side-light type has a light source element such as a cold cathode tube disposed on the lateral side of a light guide plate and uses one surface of the light guide plate as an emitting surface. Such a surface light source device has properties of obtaining an illumination flux having a relatively large sectional area in a thin structure, and is widely applied to a back lighting arrangement in a liquid crystal display.
A light scattering and guiding material is well known as a material of the light guide plate. The light scattering and guiding material includes an optical element having scattering power obtained by distributing a microscopic structure of uneven refractive index inside a transparent optical material. The light guide plate made of the light scattering and guiding material is called a light scattering light guide plate. In general, a surface light source device of side-light type adopting the light scattering light guide plate has the advantage of obtaining a high efficiency of light utilization in a simple structure.
Unless the microscopic structure of uneven refractive index inside the light scattering light guide plate is made excessively small in size, clear directivity is added to an output flux from an emitting surface. A light guide plate meeting the above requirement is called "a light guide plate with emitting directivity" in the present specification.
According to Debye's theory of scattering, the size of the structure of uneven refractive index distributed inside the light scattering and guiding material may be represented in terms of a correlation distance a. The requirement of correlation distance a.gtoreq.0.06 constitutes one practical criterion to exert clear emitting directivity in the light scattering and guiding material.
In addition to the light scattering light guide plate described above, a light guide plate having a large number of fine irregularities provided on a surface of a transparent plate to restrain total reflection is well known as the light guide plate with emitting directivity applicable to the surface light source device of side-light type. The irregularities may include numberless fine irregularities on the surface of the light guide plate itself or numberless fine particles fixed to a flat surface of a transparent plate with a light transmitting binder.
In the surface light source device of side-light type adopting the light guide plate with emitting directivity, extremely high luminance is obtained when an emitting surface is observed from a direction coincident with its directivity. However, a problem in this case is the fact that a main propagation direction, that is, "a preferential propagation direction" of an output flux from the emitting surface of the light guide plate with emitting directivity is largely deviated from a frontal direction of the emitting surface.
FIG. 1 is a graph illustrating the above fact, and angle characteristics of the intensity of output light in the surface light source device of side-light type adopting the light guide plate with emitting directivity are plotted. The conditions of measurement in this graph are as schematically shown in FIG. 2. A light guide plate 1 used in the surface light source device to be measured is made of a light scattering and guiding material having a wedge-like sectional shape. This light scattering and guiding material has a matrix consisting of polymethyl methacrylate (PMMA having refractive index of 1.492), the numberless fine particles of refractive index different from that of the matrix are uniformly distributed in the matrix.
A silicone resin material (Tospearl 120: registered trademark/manufactured by Toshiba silicone Co., Ltd.) is distributed as the fine particles in the light guide plate 1 at a rage of 0.03 wt %.
As shown in the drawing, the light guide plate 1 is sized to be 180 mm in depth as viewed from the side of an incidence surface 2, 135 mm in width, 2.5 mm in thickness on the side of the incidence surface 2, and 0.5 mm in thickness on the side of an end surface 7. A straight tube-like lamp L (a cold cathode type having a diameter 1 of 2.4 mm) is disposed at a distance of 1.0 mm from the incidence surface 2 of the light guide plate 1. The lamp L is surrounded from the rear by a reflection sheet R consisting of silver foil in order to prevent light from being scattered and lost. A silver foil sheet is disposed as a reflector 3 along a back surface 6 of the light guide plate 1. A thin air layer (having a thickness of .delta.1) is present between the silver foil sheet 3 and the back surface 6.
In FIG. 2, reference symbol M denotes a luminance meter (LS110 manufactured by Minolta, having a visual field angle of 1/3.degree. in measurement, and mounted with a close-up lens) used for measurement of luminance. In measurement with the luminance meter M, an observation of a central point P on the emitting surface 5 was made though a line of sight b at a distance of 203 mm from the central point. Then, the line of sight b was scanned by turning within a plane perpendicular to the lamp L. In the abscissa of the graph, a direction of the line of sight b is represented in terms of an output angle .phi. (.degree.). COS-corrected relative luminance values (%) to a peak value are plotted on the ordinate.
COS correction is made for compensating an emitting surface area corresponding to output light incident on the luminance meter for COS-functionally varying depending on the angle of the line of sight. COS correction is also applied to other graphs which will be described later.
As is read from the graph of FIG. 1, a sharp peak (a preferential propagation direction) is observed in a direction of an angle slightly lower than 80.degree.. In view of such a fact, a surface light source device of side-light type adopting the light guide plate with emitting directivity will need to modify a preferential propagation direction of output light from the emitting surface. In particular, when this surface light source device is applied to a back lighting arrangement in a liquid crystal display, it is necessary to modify the preferential propagation direction to a frontal direction which is the most general direction of observation.
An element called a prism sheet has been conventionally used to modify the preferential propagation direction. As a known arrangement of the prism sheet, there are a prism sheet arrangement, in which a prism surface is faced inwards (is opposed to the emitting surface), and a prism sheet arrangement, in which a prism surface is faced outwards (turns its back on the emitting surface).
FIG. 3 is a sketch showing a basic arrangement of a surface light source device adopting the former arrangement. This arrangement is provided by adding the prism sheet to the arrangement shown in FIG. 2 (an illustration of the conditions of measurement), and a reference numeral of each element is also used in common.
A prism sheet 4 is disposed on the outside of an emitting surface 5 of a light scattering light guide plate 1 with emitting directivity having a wedge-like sectional shape. The prism sheet 4 includes a light transmitting sheet having a surface provided with a large number of prism rows at fine pitches and a flat surface 4e. Each prism row is composed of a pair of slopes 4a, 4b.
A plastic material such as polycarbonate is usually used as a material of the prism sheet. As a matter of convenience, a distance between the prism sheet 4 and the emitting surface 5 and a pitch of prism rows are exaggerated in FIG. 3 and other drawings.
When the surface light source device is applied to a back lighting arrangement in a liquid crystal display, a well-known liquid crystal display panel is disposed on the further outside of the prism sheet 4. Since the light guide plate 1 has a wedge-like sectional shape, it is advantageous in improving a luminance level and ensuring uniformity in luminance. An action based on such a shape of the light guide plate is disclosed in Japanese Patent Laid-open No. 7-198956, for instance.
Light supplied from the lamp L to the light guide plate 1 is guided toward an end surface 7 on the thin side while being affected by a scattering action and a reflecting action in the light guide plate 1. In the process, illumination light is outputted little by little from the emitting surface 5. As described above, the output light from the emitting surface 5 of the light guide plate 1 with emitting directivity has a clear preferential propagation direction 5a. The preferential propagation direction 5a is inclined at an angle of about 60 to 80.degree. with respect to a normal extending from the emitting surface 5, as illustrated in the graph of FIG. 1.
The output light from the emitting surface 5 in the preferential propagation direction 5a is introduced into the inside surfaces 4a, 4b of the prism sheet 4 and is outputted from the outside surface 4e to around a frontal direction. As a result, the preferential propagation direction is modified. A description of this modifying action will be given as follows with reference to FIG. 4.
FIG. 4 is a view for explaining the behavior of light in a section orthogonal to the lamp L in the arrangement shown in FIG. 3. In this case, "a direction orthogonal to the lamp L" means "a direction perpendicular to a running direction of the lamp L", that is, "a direction perpendicular to a running direction of the incidence surface 2". Hereinafter, it will be simply referred to as "orthogonal to the lamp". Similarly, "a direction parallel to a running direction of the lamp L", that is, "a direction parallel to a running direction of the incidence surface 2" will be referred to as "parallel to the lamp".
The prism sheet 4 is disposed inwardly along the emitting surface 5. This arrangement is briefly called an arrangement "with grooves inwards". Each prism unit forming the prism surfaces has an isosceles triangular section. Its apex angle is represented by .phi.3. A direction of incidence is given by an arrow L', and a preferential propagation direction of an output flux from the emitting surface 5 is represented in terms of an output angle .phi.2. As described above, the angle .phi.2 is generally sized to be in the range of about 60 to 80.degree..
The light guide plate 1 having PMMA matrix has refractive index of about 1.5, and an incidence angle .phi.1 of light propagating from the inside to the emitting surface 5 is sized to be slightly lower than 40.degree.. A beam defined by such an incidence angle .phi.1 and an output angle .phi.2 is represented by a typical beam B1. In general, the typical beam corresponds to a beam propagating in a preferential propagation direction.
After the typical beam B1 outputted from the emitting surface 5 makes a straight propagation through an air layer AR (having refractive index n0 of 1.0), this typical beam B1 is incident on the slope 4a of the prism sheet 4 at substantially right angles therewith and is affected by a refractive action to some extent. It is to be noted that the beam B1 is incident on the slope 4b opposite to the slope 4a at a relatively small rate.
The typical beam B1 further makes a substantially straight propagation through the prism sheet 4 up to its slope 4b, and is regularly reflected (totally reflected) by the slope 4b, resulting in being incident on the flat surface 4e of the prism sheet 4 from the inside. When the prism apex angle .phi.3 is appropriately designed according to the output angle .phi.1 and the refractive index n2 of the prism sheet 4 and so on, the incidence angle with respect to the flat surface 4e comes to be about 0.degree., and a beam 4f leading to around a frontal direction (at an angle .phi.4 of about 90.degree.) is generated.
In this manner, the preferential propagation direction is completely modified to the frontal direction. In this arrangement, the prism sheet 4 functions as a deflecting element for an output flux represented by the typical beam B1, whereas it hardly functions as a converging element which narrows diffusion of the propagation direction of an output flux from the emitting surface 5. That is, a flux is sufficiently deflected whereas an action for converging the flux to improve a degree of parallelization in a propagation direction is hardly expected.
Although a propagation of light from a medium of low refractive index to a medium of high refractive index is applied to incidence of light on the prism sheet 4 through the air layer AR, the light is incident on the slope 4a at substantially right angles therewith as described above. Under these conditions, a converging action is hardly caused. The converging action is not caused even through a process of propagation in the prism sheet 4 or a process of reflection by the inside surface of the slope 4b.
In output of light from the flat surface 4e of the medium of high refractive index, diffusion of light occurs on the contrary. However, output of light at substantially right angles causes a low diffusing action from the reasons similar to those in case of incidence of light on the slope 4a.
An arrangement with prism surfaces facing outwards has been proposed in order to fulfill the converging action in the prism sheet 4. FIG. 5 is a sectional view for explaining the behavior of a typical beam in case of adopting the above arrangement.
Reference numerals 4c, 4d respectively denote slopes forming the outward prism surfaces, and 4g denotes an inward flat surface. The flat surface 4g is parallel to the emitting surface 5. This arrangement is briefly called an arrangement "with grooves outwards". A prism row serving as a prism unit of the prism sheet 4 having an apex angle .phi.5 has an isosceles triangular section. With respect to inclination angles .phi.6, .phi.7 of the slopes 4c, 4d, .phi.6 is sized to be equal to .phi.7.
Similarly to the case of FIG. 4, a direction of incidence is given by an arrow L', and a typical beam is represented by B2. The typical beam B2 is incident on the emitting surface 5 at an inside incidence angle .phi.1 slightly lower than 40.degree., and most of the beam is outputted to an air layer AR (having refractive index n0 of 1.0). An output angle .phi.2 in this case is sized to be in the range of about 60 to 80.degree., as described above.
After the typical beam B2 outputted from the emitting surface 5 makes a straight propagation through the air layer AR, this typical beam B2 is obliquely incident on the flat surface 4g of the prism sheet 4, and is outputted from the surface 4c or 4d of the prism sheet 4 through refractive paths P1, P2 as shown in the drawing. When the prism apex angle .phi.5 is appropriately designed according to the output angle .phi.2 of the beam from the emitting surface 5 and the refractive index n2 of the prism sheet 4, a propagation direction of output light 4f may be modified close to a frontal direction.
According to this arrangement, an action for converging a flux to a frontal direction may be fulfilled by (1) the fact that light is incident on the flat surface 4g parallel to the emitting surface 5 and (2) a function of the prism surfaces 4c, 4d as a kind of convex lens array.
The fact described in (1) may be generalized as follows.
If a light transmitting element having a flat surface is disposed such that the flat surface faces inwards and is parallel to the emitting surface 5, a flux obliquely incident on the flat surface is affected by a kind of converging action. A description of this fact will be simply given with reference to FIG. 6.
Referring to FIG. 6, a light transmitting element 10 having a flat surface 11 is disposed along an emitting surface 5 of a light guide plate 1 (having refractive index of 1.492) with emitting directivity as shown in the graph of FIG. 1. This light transmitting element 10 includes a flat plate (having refractive index of 1.492) made of PMMA, for instance. The flat surface 11 of the flat plate 10 is parallel to the emitting surface 5, and an air layer AR (having refractive index of 1.0) is present between the flat surface 11 and the emitting surface 5.
Now, with respect to a flux represented by a typical beam B10, let's pay attention to a "a partial flux" propagating through the air layer AR in the angle range of 20.0.degree. around the preferential propagation direction. It is estimated from the result of actual measurement in FIG. 1 that such a partial beam is considered to represent most of an output flux from the emitting surface 5. Beams on both sides of the partial flux defined by this angle range are represented by B11, B12. An attempt to trace the beams B10, B11, B12 is made under the conditions of the above refractive index according to Snel's rule.
The results are shown in FIG. 6. The diffusion of a flux in the angle range of 20.1.degree. in the propagation through the air layer AR is narrowed to that in the angle range of 6.9.degree. through a process of refraction at the time of oblique incidence of light on the flat surface 11. In other words, the diffused state of the beams B10, B11, B12 (see reference symbols C to C") in the light guide plate 1 is recovered.
That is, when the flat surface of the light transmitting element formed on the surface of a medium having refractive index higher than that of the air layer AR is disposed in parallel to the emitting surface 5, this flat surface fulfills an action of recovering directivity once reduced due to an escape of light to the air layer AR.
A group of beams denoted by reference numerals C', S represents a situation of beams traced on the basis of a certain incidence point Q on the flat surface 11, and the converging action is more clearly understood from this situation.
While the arrangement "with grooves outwards" shown in FIG. 5 is excellent in utilization of such a converging action, it has difficulty in obtaining a flux deflecting action, in comparison with the arrangement "with grooves inwards" shown in FIG. 4. That is, in the arrangement "with grooves outwards", it is not possible to utilize reflection by the inside surface such as the slope 4b in FIG. 4, and a flux is deflected only by a refracting action in both surfaces of the prism sheet.
That is, under the conditions of the refractive index (refractive index of about 1.5) of generally available materials for the prism sheet and the light guide plate, it is difficult to lead a flux to a frontal direction, even if the prism apex angle .phi.5 is adjusted.
A graph of FIG. 7 shows the result of actual measurement illustrating this difficulty. With respect to the arrangement "with grooves inwards" and the arrangement "with grooves outwards", angle characteristics of output light intensity are given under the above conditions of measurement (see a description related to FIG. 2). In a graph I, angle characteristics of output light intensity are plotted when a prism sheet having a prism apex angle of 70.degree. in the arrangement "with grooves outwards" is added to an arrangement corresponding to the graph of FIG. 1.
On the other hand, a graph II shows angle characteristics of output light intensity when a prism sheet having a prism apex angle of 66.degree. in the arrangement "with grooves inwards" is added to the arrangement corresponding to the graph of FIG. 1. The ordinate is graduated so as to show relative luminance (%) on condition that a peak in the graph I is defined as 100.
As is read from a comparison between both the graphs, the arrangement "with grooves outwards" in the graph I is substantially excellent in function of convergence, while being inferior in function of deflection to a frontal direction, in comparison with the arrangement "with grooves inwards" in the graph II. A preferential propagation direction of illumination light is deviated from a frontal direction by 20.degree. or more. Such a tendency is generally observed without being limited to the above case.
Therefore, two prism sheets in the arrangement "with grooves outwards" have been generally layered. A graph in FIG. 8 shows the result of actual measurement of angle characteristics of output light intensity when "two prism sheets in layers" are applied. The conditions of measurement are similar to those in FIG. 7.
A graph III shows angle characteristics of output light intensity when two prism sheets, i.e., a prism sheet having a prism apex angle of 66.degree. and a prism sheet having a prism apex angle of 90.degree. in the arrangement "with grooves outwards" are added in layers to the arrangement corresponding to the graph of FIG. 1. A graph II shows angle characteristics when the prism sheet having the prism apex angle of 66.degree. as described in FIG. 7 is used in the arrangement with "grooves inwards". The ordinate is graduated so as to show relative luminance (%) on condition that a peak in the graph III is defined as 100.
As is read from both the graphs, two-layered arrangement "with grooves outwards" is not only substantially excellent in function of convergence, but also in function of deflection to a frontal direction, in comparison with the arrangement "with grooves inwards" in the graph II. In other words, the two-layered arrangement "with grooves outwards" eliminates a deficiency of the function of deflection in the single arrangement" with grooves outwards" shown in FIG. 7.
However, this method arises a problem caused by an overlap of fine grooves of the two prism sheets. That is, two prism sheets in layers have a tendency to generate moire fringes. Further, a fixed phase relation between repetitive grooves of two prism sheets is required for obtaining higher efficiency of light utilization.